Automatic gain control



April 18, 1933. T I A SMH-H l AUTMATIC GAIN CONTROL Filed oct. 29, 1929sheets-sheet l INVENTOR THEODDRE A. SMlTH BY 7 illu .Nhk

ATTORNEY AUTOIVIAT IC GA IN CONTROL Filed oct. 29, 1929 3 sheets-sheet 2Jig. 5

rau mmm a I l lI I l l I I I I I I I I I l l I I I I l I I I l I Il IIAI 1 INVENTOR THEODoRE'A-sM1TH April 18, 1933. T. A- SMH-H 1,904,552

AUTOMATFIC GAIN CONTROL Filed Oct. 29, 1929 3 Sheets-Sheet '5 mvENToRTHEODORE A SMITH' ATTOR N EY lzo Patented Apr. 18, 1933 PATEN T OFFICEIHEODORE A. SMITH, OF RIDGEWOOI), NEW JERSEY, ASSIGNOR T0 RADIOCORPORATION 0F AMERICA, A CORPORATION OF DELAWARE I AUTOMATIC GAINCONTROL Application filed October 29,'

My present invention relates to an' automatic gain or` amplificationcontrol, and more particularly to an lmprovement 1n a .gain controlcircuit of the type disclosed in my co-pending application Serial No.310,- 497 filed October 5th, 1928.

In the aforesaid co-pending application there has been disclosed anaudio frequency amplifier whose gain varies inversely with the inputlevel so that a sudden increase in input is limited to a predeterminedconsta-nt output. Briefly, the manner in which this is accomplished isby varying the anode load of an amplifier tube by automatically ladjusting the anode impedance of a control tube in a predeterminedmanner, said anode impedance being in parallel with the amplifier anodecircuit, the uni-directional component of a rectified ortion of thealternat#f ing'input current to e amplified being empl'oyed to securethe adjustment. Thus, the change in the anode load of the amplifieralters its amplification.

Now, by further experimentation various improvements have been made uponthe invention in the said co-pending application, it being observed thatthe present arrangement is primarily designed for, and has for its mainobject, a use where very sudden increases in input voltage, (due forexample, to a very sudden increase in volume during a musical program),may cause overloading to occur, and additionally, to augment rather thanto supplant manual operation of a gain control, since 'for suddenincrease in input voltage the arrangement is faster than manualoperation.`

Another important object of the invention is to provide an audiofrequency amplifier including controlling equipment broadly comprisingan amplifier, a rectifier, a control tube and a time control and filterarrangement for controlling the speed at which the impedance of thecontrol tube varies, the control equipment including a plurality ofaudioamplifier tubes, the plate circuits of the control tube and the first ofsaid plurality of amplifier tubes being inductance coupled to maintainthe plate voltages constant regardless of the impedance of 1929. SerialNo. 403,281.

the control tube whereby the dynamic characteristic of the saidplurality of amplifier tubes remain more constant during operation ofthe control tube.

Another object of the invention is to provide lan amplifier circuithaving a predetermined'ratio between the input energy applied to theamplifier and the output energy derived therefrom which comprises acontrol equipment including a plurality of amplifier tubes, the input tothe last tube being obtained from a drop across a part of a loadresistance connected in the output circuit of a control tube, the'latterbeing supplied with a rectified portion of the altern ating current tobe amplified, there being a condenser connected in series with said loadresistance and forming the alternating current output circuit of thecontrol tube, the system thereby being less 'likely to cause distortion,since high resistance is always in the circuit regardless of the plateimpedance, of the control tube.

Still another object of the invention is to provide an audio amplifierwhose output is maintained substantially constant while the input isvaried, means for accomplishing the latter result, said means beingdesigned to respond quickly or slowly, as desired.

Other objects of the invention are to improve generally the simplicityand efiiciency of automatic gain control devices for amplifiers, and toespecially provide an automatic gain control for an audio amplifier,which is durable and reliable in operation, and economical tomanufacture.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims, the invention,itself, however, as both its organization and method of operation willbest he understood by reference to the following description taken inconnection with the drawings in which I have indicated diagrammaticallyone circuit organization whereby my invention may be carried intoeffect.

In the accompanying drawings,

Fig. 1 is a lschematic diagram showing the equipment essential'to anamplifier embody ing the present invention,

in which like characters of reference ind1 'cate the same4 parts in thediii'erent figures,

.A second im edance there is fed into the apparatus, as shown in Figs. 1and 2, at input a varying alternating input. A portion of this input iss, fed directly into the first amplifier tube 2 of the controlequipment. Another portion of the alternating linput is fed into anamplifier tube 3 of the controlling equipment, and thence into arectifier 4. It is to be understood that, if desired, the amplifier tube3 may be omitted.

The uni-directional component of the rectiier current is then fed intothe time control and filter device 5.' The latter controls the speed atwhich the bias on the control electrode of control tube 6 Eis varied bythe changes in the uni-directional component of the rectified current. v

Control tube 6 has its anode-cathode current supplied throu h the anodeimpedance 7 The first ampli er`tube2 of the control equipment hasconnected between its anode and cathode a resistance tapped at a point16, the latter dividing the resistance into two portions R8, Rg. Theimpedance 7, in this case a choke coil, is connected in series with theportion R2 R1, and in series with a condenser 10, the resistance R1having one of Iits/terminals connected to point 16.

The-anode of tube 2 is connected in series with the portion R3 throughacondenser 14. 15, in this case also a choke coil, 1s connected in theanode circuit of the tube 2, anode current being supplied therethrough.The input to the last amplifier tube 2 is obtainedfrom the drop acrosspart of the grid leak resistance 13.

which together with a series condenser 12, the latter having one of itsterminals connected' to point 16, forms the alternating current outputcircuit of the amplifier. It will also be observed that the portion ofthe input which is fed directly -to the control amplifier tube 2 rstpasses through the re-Vv sista-nce Rz, R3. i i r` In Fig. 2 I haveshownV in'gneater detail the connectionsand apparatus employed in theconstruction schematically shown in Fig. 1. The amount of the inputenergy applied to the controlling equipment amplifier 3 is varied bymeansronf armanually controlled resistance 8, the output of theamplifier tube 3 being fed, by means of an audio transformer 9, intorectiiier tube 4, the latter having a cathode of the heater type.

byA means of a resistance' The control electrode and anode oftherectifier 'tube` 4 are connectedtogether forming a twoelementArectiier,` used so as to produce a more nearly perfectrectilinear characteristic. The other portion of the input alternatingenergy is fed through a condenser 20 to the irst amplier tube 2 of thecontrol equipment. Y

The time control, and filter device 5 shown diagrammatically in Fig. 1preferably comprises, 31, choke coil 32, variable resistance R andcondenser 33. With a suiiciently large input the negative grid bias ofcontrol tube- 6 is reduced so that controlling action results. That is,under those circumstances the internal impedance of the tube 6 isreduced, and, as its anode current passes through the anode impedance orchoke coil 7 A of the rst control ampliier tube, a change occurs in theeffective value of the anode impedance or load of tube 2`which latterconsists of the impedance 15, and resistances R5, R3, in parallel withthe control tube 6. The latte effect lessens the amplifying eii'ect oftube onpthe input supply to it through condenser The speed of control isvaried by Varying resistance R. The reason why variation of resistance Rwill control the speed of opera- 'lol tion of the apparatus maybeexplained as.

follows: When a high input which has been applied is sudderly removed,and assuming that the condensers 31 and .33 remainl charged, the chargeon condenser 31 can only discharge through high resistance 30, if theimpedance of therectifier tube 4 is high. Condenser '33 can onlydischarge through the series combination of variable resistance R, choke32, and resistance 30, since the internal impedance of the control tubeis also high. p When the rectier impedancefis low, as

for example, when a large `Voltage is sup,-A plied by transiormer 9 torectifier 4 thenthe charge on 'condenser 31 can leak ofi' acrossHowever, ncondenser 33 willstillhave to discharge through resistance Rwhich .may be varied to control its speed of discharge- Hence, the speedat which the bias on th control electrode of the tube 6 may be-varied,is secured by 'the manipulation of resist-lV ance R. Y'lVhen a stronsignal is applied. to the controlling amplifier tube 3, condenser-s 31and 33 will become highlyv charged and Ahecause of the reducedvr controlelectrode -or negative grid bias'on wijl have its impedance reduced.this strong signal is suddenly cut oif the im pedance of rectifier4,Wil1 increase rapidly.

Condensers 31 and- 33 ywill retain their charge until resistors 30 and Rdischarge them.

As a result, um com 1 mbe gris bias win tube 6, vcontrol tube .6 Y

mains substantially constant.

remain nominally reduced and the amplification of control amplifier tube2 low. A resistance R4, the latter being of a higher order of magnitudethan resistance 30, is disposed across the anode-cathode circuit ofrectifier tube 4 and decreases the time of discharge in this case, andmakes the controlling action more uniform. Of course, the speed ofaction can still be manually controlled by adjusting the resistance Rwhich determines the discharge rate of condenser A milliameter A isdisposed in the output circuit of control tube 6 t`o indicate itsoperation. The amount of control is indicated by the milliameter whichis mounted on the front panelof the receiver. When the latter is readingzero, the control tube is not acting; when it reads 2.5 milliamperes,then the control tube is acting to its full capacity. The meter, inother words, reads the control tube plate current.

As the control device does not operate from the output voltageA (as inthe case of certain radio frequency automatic gain controls) it isnecessary to carefully adjust the various resistances, so that theoutput re- The resistance R1 prevents over control which will occur ifthe control tube plate impedance becomes too low compared to R2. If R1is too high, insufficient control will be obtained. The resistance R3 isplaced in the amplifier plate circuit to prevent distortion which wouldotherwise occur when the control tube plate impedance becomes too low.

As indicated heretofore, the hi h resistance R4 is employed to allowthe4 lter condenser charge to leak by the' rectifier tube when thevoltage employed is low and the rectifier impedance is high. When a highvoltage input is applied, the rectifier impedance is low and thisresistance is not needed. The effect of this phenomenon is to allow thecontrol to act rapidly in cutting ofi the gain when high inputs areapplied but to tend to hangover after the high input has decreased.

The resistance R,t has been selected to partially prevent this. It hasbeen noted that when a high input is applied, and then removed, themilliameter reading increases rapidly, decreases rapidly lat first, andbecomes slower and slower as the reading decreases. The time actioncontrol'allows individual variations, or sudden checks to be controlledby making the action fast; or to control merely the general level bymaking the action slow.

It may be found necessary to place a grounded shield between therectifier tube 4 and the control tube 6 to prevent distorted signalsfrom getting into the output amplifier. The amplifier is 'designed tooperate -between a 200 ohm input and 500 ohm load,

which may be readily adaptedl to other input and output impedances. v.

The system is, further, provided with a switch, generally denoted by S,that facilitates connecting the input directly to the output withoutbeing controlled. This may be desirable if for any'reason the systemshould cease to function properly. The systempmay then be disconnectedand the manual control of the input utilized until the trouble isrectified.

The main plate battery employs a voltage of 135 volts, the voltagesindicated in Fig. 2 being those which have been found to be satisfactoryin practice. Resistances 20 are used to lobtain correct voltages for thetube filaments. The .positive A connection is preferably grounded andconnected to the positive C and negative B.

The control acts by shunting the plate iinpedance of a tube across aportion of a coupling impedance in an impedance capacity coupledamplifier. The plate impedance of the tube is changed in accordancevwith the input level by amplifying some of the input, rectifying it, andapplying the direct current component as grid bias for the. controltube. It will be noted that only a single stage of amplification isemployed before rectification instead of the two stages of amplificationas disclosed in my abovementioned co-pending application. Onestage hasbeen found sufiicient to deliver the necessary output to operate thecontrol tube satisfactorily. p

The actual amplifier from which the output is taken is the equivalent ofa two stage amplifi-erand has a gain of about 30 decibells, when thecontrol is not acting. Its gain may be varied manually, of course, bythe control 13. The control .will begin to ac t when an input of about15 millivolts is applied, when the control gain is set at maximum. Ifthe'corftrol gain is reduced, higher inputs are required before thecontrol acts. The control action has little effect on the frequencyamplification, as is well shown in Fig. 3 which graphically illustrates'the `characteristics for full and no control.

frequency characteristic in Fig. 3 indicates that there is practicallynov change produced The in relative amplification of high and low agesfor various inputs and different degrees of control. The input-outputcurve in Fig.v

v 4, shows that without control, the output increases proportionatelywith Ainput as far as the curve has been plotted. With control majordifferences between the present ar" rangement and that described in mycopending application involves the factthat the plate circuits of thecontrol and amplifier tubes of the present arrangement are inductancecoupled rather than resistance coupled.

This tends to maintain the plate voltages constant regardless of theimpedance of the control tube. This'results in the maintenance of thedynamic characteristic of the tubes more constant during operation ofthe control tube 6.

Again, another important difference and improvement involves the factthat the input to the last amplifier tube 2' of the control equipment isobtained from the drop across part of the load resistance 13 whichtogether with series condenser 12, forms the alternating current outputcircuit of the amplifier. By the use of this improved method the systemis less likely tol cause distortion since a resistance of at leastv48,000/ohms is always in the circuit regardless of the plate impedanceof the control tube 6.

To analyze these differences in more detail it will be noted that in anordinary amplifier circuit, without the volume control rinciple, for thetube 2', would consist o the tube, its plate load impedance 15', a gridleak 13', and a stopping condenser12'. In this arrangement, the totalvoltage drop across the impedance 15' would be applied to the idy of thesucceeding tube, 2. (See Fig. 5. If now a resistance is shorted lacrossthe plate load 15', the amplification of tube 2' will vary according tothe value of the load. If this resistance is made to vary inversely asthe amplitude of the applied signal voltage, a more or less uniformoutput will be obtained from a varying input. y

If such a load is introduced, inthe form of the plate impedance of avacuum tube, it will be necessary to isolate its plate circuit from thegrid circuit of the tube 2. (See Fig. 6.) This is done by the (gridblocking condenser 12'. It will be note that the anodes of tubes 2' and6 are in parallel'and that if, a larger positive bias is applied to thegrid of tube v6. (caused by a strong signal being applied and hence alarge rectified component being present), thevo'ltage drop across 15'will increase, which action will reduce the anode voltage on tube 2.This is essentiallythe circuit described in my aforementioned co-pendingpatent application. Such variation in plate voltage will causedistortion of the output wave form.-

If however, the plate voltage supply' of vcontrolled tube 2', andcontrol .tube 6, can

producing determined be made independent, this distortion will no longerbe present. Furthermore, individual tube adjustments can be made withoutaffecting other tubes. The present improved circuit is analyticallyshown in Fig. v7. Here, 12 and 10' are plate blocking condensers, andthe tubes 2 4and 6 are supplied through individual anode loads, .15' and7' respectively. These loads may be either resistances or inductances,preferably the latter. Load 15' may be considered as the couplingimpedance between tubes 2'y and 2.

It has been found in practice that the plate impedance of tube 6 becomeslower more rapidly than it should for maintaining constant output. Toavoid this, the plate impedance of tube 6 is shunted across only aportion of the plate load of tube 2'. This is accomplished by means ofthe potentiometer R2, R3. (See Fig. 8.) Resistance R1 is used to preventthe plate circuit impedance of tube 6 from decreasing to too low avalue.

It is to be understood that while the invention is especially adaptedfor use with audio frequency currents, it may, if desired be utilized ina radio frequency amplifying system. Furthermore, while I haveindicated' andY described one arrangement for carrying my inventlon intoeffect, it will be apparent to one skilled in the art that my inventionis by no means limited to the particular organization shown anddescribed, loutu that many modifications may be em- ,.ployed, withoutdeparting from the scope of clalms.

What I claim is:

my. inventionl as set forth in the appended 1. In an amplifying system,an electron discharge tube comprisin an anode. a cathode and a controlelectrode, means to apply alternating current energy variations to saitube, means for rectifying a portion of the alternating current energyto be amplified v by said tube, means including a control tube to varythe e'ective anode load of said first tube according to the amplitude ofthe rectifed current, and a common path, including a pair ofinductances, for applying a positive potential Vto the anodes of thefirst tube and the controLtube.

2. In an amplifying system, an'ampliier tube, means to apply alternatingcurrent to the control electrode of said tube', means for a directcurrent whose value is by the amplitude of.- said alternating current,means including 4an electron discharge tube to apply said direct currentto said amplifier tube to control'the anode potential thereof, and meansfor applying a desired positive potential to the anodes of the amplifiertube and said electron discharge tube, said means including a choke coilconnected to each of said last mentioned anodes.

3. An alternating current amplifier comprisin a tube having an anode,cathode, and contro electrode, an impedance in the anode circuit of saidtube, another tube shunting said impedance, means for applying a portionof the alternatiu current to said first tube, means for rectiyinganother ortion of said alternating current, means for applying tothecontrol electrode of the tube shunting said impedance, the directcurrent component of said rectified alternating current and acapacitative path connecting the anode of said first tube with the highpotential side of said impedance.

4. An audio frequency amplifier comprising a tube, a choke coilconnected te the anode of said tube, a control tubel shunting said chokecoil, means for applying a portion of an alternating current to beampli-- fied to the control electrode of said first tube,

means for rectifying another portion of said alternating current, meansfor applying the uni-directional component of the rectified current tothe control electrode of said control tube whereby the anode impedanceof said first tube is varied in accordance with the strength of therectified current.

5. An audio frequency amplifier comprising a tube, a choke coilconnected to the anode of said tube, a control tube shunting said chokecoil,'means for applying a portion of an alternating current to be amlified to the control electrode of said `rst tube, means for rectifyinganother portion of said alternating current, means for applying theuni-directional component of the rectifiedcurrent to the controlelectrode of said control tube whereby the anode impedance of said firsttube is varied in accordance with the strength of the rectified current,and a capacitative path connecting the high potential side lof saidchoke coil with the anode of said rst tube.

6. In an amplifier system, an amplifier tube, means to apply analternatin current to be amplified to the input of said tube, meansorrectifying a portion of the alternating current to be amplified bysaid tube, a'

control tube to vary the effective anode lo'ad of said amplifier tubeaccording to the amplitude of the rectified current, means to controlthe-s eed at Awhich said anode load is varied, andp a capacitative pathconnecting the anodes of the control tube and amplifier tube.

7. In an amplifier system, an amplifier tube, means to. apply analternating current to be amplified to the input of said tube, means forrectifying a portion of the alternating current to be amplified by saidtube, a control tube to vary the effective anode load of said amplifiertube according tothe amplitude of the rectified current, means tocontrol the speed at which said anode load is varied, a capacitativepath connecting the anodes of the control Vtube and amplifier `ternatingcurrent, a control tube to ap tube and la resistor connected between a.point on the capacitative path and the cathodes of the control tube andamplifier tube.

8. In an amplifier system, an amplifier tube, means to apply analternating current to be amplified to the input off' said tube, meansfor rectifying a portion of the alternating current to be amplified bysaidtube, a control tube to vary the effective anode load of saidamplifier tube according to the amplitude of the rectified current,means to control the speed at which said anode load is varied, acapacitative path connecting the anodes of the control tube andamplifier tube, a second amplifier tube, and an adjustable path,including a condenser, connected between the id of the second amplifier1tube and a point on said capacitative pat 1.

9. An audio frequency amplifier comprising a multi-electrode amplifiertube, a choke coil in the anode I'circuit of said amplifier' tube, amulti-electrode control tube, a choke coil connected to the anode ofsaid control tube, means for connecting both said choke coils to asource of positive potential, means for applying a portion of analternating current to be amplified to the control electrode of saidamplifier tube, means' for rectifying another portion of saidalternating current, means for applying the uni-direc-4 tional componentof the rectified current to the control electrode of said control tubewhereby the anode impedance of said amplifier tube is. varied, inaccordance with the strength of the rectified current, means forcontrolling the speed at which said anode impedance varies, said meansincluding a variable resistance and a condenser, a capacitative pathconnecting the high potential sides of said choke coils, a resistorconnected between a oint on the capacitative path and the catho es ofthe control tube and said amplifier tube, a second amplier tube, and anadjustable path, including a condenser, connected between the grid ofthe second amplifier tube and point on said` capacitative path.

10. In an amplifying system, an electron discharge tube including aplurality of electrodes, means to apply alternating current to thecontrol electrode of' said tube, means for producing va ldirect currentwhose value is determined by the amplitude of said all saiddirectcurrent to said `first tube to ciin-7 trol the anode potential thereof,and a network including lumped inductive and condensive impedancesconnecting the anode and cathode of said first tube with the anode andcathode of said control tube.

11. In an amplifying system, an electron discharge tube including aplurality of electrodes, means to apply alternating current to thecontrol electrode of said tube, .means 'ode of said first tube with `forimpressing on t directcurrent to said first tube to control the anodepotential thereof, a network in- 4cluding lumped inductive andcondensive anode and caththe anode and cathode of said control tube, andmeans to control the rate at which said anode potential is varied. 1,

` 12. In an amplifying system, an electron discharge tube including aplurality of electrqdes, means to apply alternating current to thecontrol electrode of said tube, means for producing a direct currentwhose value impedances connecting the -is determined by the amplitudeofsaid alternating current, a control tube to apply sald direct currentto saidl rst tube to control the anode potential thereof, a networkincluding lumped inductive and condensive impedances connecting theanode and cathode of said first tube with the anode and cathode of saidcontrol tube, a second ampliier tube andan adjustable impedance path,including a condenser connecting the grid of said second ampliier tubeto a point on said network.

13. In combination, in an automatic gain control circuit for anampliier, an amplifier tube having its input electrodes connected to asource of energy to be amplified, a gain control tube, a common path forconnecting the anodes of said amplifier and control tubes to a source ofpositive potential, means for impressing on the input of-said controltube control energy derived from said source whereby the internalimpedance of said control tube is adjusted in a predetermined P mannerwith respect to variationsl of said source energy, and a capacltativepath, independent of said common path, connecting .the anodes of saidcontrol tube and ampliiier tube. e

y 14. In combination, in an automatic gain control circuit for anamplifier, an amplifier tube' having its input electrodes connected to asource of energy to be amplified, a gain control tube, a common path forconnecting the anodes of said amp tubes to a source of positlvepotential, means e input of said control tube control energy derivedfrom said source whereby the internal impedance of said control tu e isadjusted in a predetermined manner with respect to variations of saidsource energy, a capacitative path, independent of said common path,connecting the anodes of said contro tube and amplifier tube, .and aresistor connected between a inton said capacitative path and the Icathodes of said amplifier and control tubes.

15. In combination, 1n an automatic gain control circuit for anamplifier, an amplifier tube having its input electrodes con- `andamplier tube,

lier and controlA nected to a source of energy to be amplified, a gaincontrol tube, a common path for connecting the anodes of said amplifierand control tubes to a source of positive ypotential, means `forimpressing on'the input of said-control tube control energy derived fromsaid source whereby the vinternal imp'edance of'said control tube isadjusted in a predetermined manner with respect to, variations of saidsource energy,'a capacitati've path, independent of said common path,connecting the anodes of said control tube said capacitative pathincluding a resistor and a second resistor connected between anintermediate point of said last mentioned resistor and the cathodes ofsaid amplifier andcontrol tubes.

16. In combination, in anautomatic gain `control circuit for anamplifier, an amplifier tube having its input electrodes coGnnected.

to a source of energy to be. amplified, a gain control tube, a commonpath for connecting the anodes of said amplifier and control tubes to asource of positive potential, means for impressing on the input of saidcontrol tube control energy derived from said source whereby theinternal impedance of said control tube is adjusted in a predeterminedmanner with respect to variations of said source energy, a capacitativepath, independent of said common path, connecting the anodes of saidcontrol tube and amplilier tube, a second amplifier tube, a resistorconnecting the cathodes of said first ampliier tube and control tube toa. point on said capacitative path, and an adjustable impedance pathconnecting the grid of said second iamplifier tube and said lastmentioned om 17. In an amplifying system, an electron discharge tubecomprising an anode, a cathode and a control electrode, means to applyalternating current energy variations to said tube, means for rectifyinga portion of the alternatin current energy to be ampliied by said tumeans including a control tube to vary the efective anode -load of saidfirst tube according to the amplitude of lthe rectified current, acommon path, for apto said tube, means for rectifying a portion of thealternating current energy to be ampliiied by said tube, trol tube tovary the eiective anode load of said first tube according to theamplitude of the rectified current, a common path, including a pair ofinductances, for applymeans including a con- Y ing a positive potentialto the anodes of the first tube and the control tube and a switchmechanism for disconnecting o r connecting at Will the said rectifyingmeans and control tube and the said lirst tube.

19. In an amplifying system, an electron discharge tube comprising ananode, a cathode and a control electrode, means -to apply alternatingcurrent ener variations to said tube, means for rectifylnga portion ofthe alternating current-energy to be amplified by said tube, meansincluding a control tube to vary the effective anode load of said firsttube according to the amplitud' of the rectified current, a common pathfor,

applying a positive potential to the anodes of the rst tube and thecontrol tube, a load circuit coupled to the anode of said rst tube,manually operable means for varying the transfer of energy between theanode circuit of said first tube and said load circuit, and a switchmechanism for disconnecting or connecting at Will the Said rectifyingmeans and control tube and the saidl first tube.

20. In an amplifying' system, an electron discharge tube comprising ananode, a cathode and-a control electrode, means to apply alternatingcurrent energy variations to said tube, means for rectiying a portion ofthe alternating current energy to be ampliied by said tube, meansincluding a-control tube to vary the effective anode load of said firsttube according to the amplitude of the rectified current, a common path,including a pair of inductances, for applying a positive potential tothe anodes of the first tube and the control tube, and a visual vcurrentindicator connected in the anode circuit of said control tube. l y

THEODORE A. SMITH.

